The auditory system comprises ascending pathways that transmit information from the ear to higher levels for perception, and descending pathways that can modify how that information is processed at each auditory nucleus. Descending projections from auditory cortex to the inferior colliculus of the midbrain play a role in a wide range of auditory functions, including selective attention, plasticity and learning, and understanding speech in noisy environments. Recent progress indicates that cholinergic cells (neurons that use acetylcholine as a neurotransmitter) play an important role in many of these functions. Many of the cholinergic effects are mediated through interactions with GABAergic cells (neurons that use GABA as a neurotransmitter) in the inferior colliculus, but further understanding has been hindered by questions regarding the underlying neural circuitry. The present proposal focuses on possible connections between the cholinergic and descending systems to a recently discovered GABAergic pathway from cells of the inferior colliculus to the medial subdivision of the medial geniculate body. One aim is to identify the locations of these cells across the functional subdivisions of the inferior colliculus cells. The second and third aim determine whether these cells receive cholinergic or auditory cortical inputs. The study will be conducted in guinea pigs, a well-researched model for many aspects of auditory function. The experiments will use multilabeling anatomical techniques, including fluorescent neuronal tracers for labeling specific circuit elements and immunochemical techniques for identifying the neurotransmitters associated with those elements. Coordinated studies with both light and electron microscopes will allow for the direct identification of synapses, a necessary step for identifying neuronal circuits. The results will provide an important step in characterizing excitatory and inhibitory midbrain circuits and their relationships to ascending and descending auditory pathways as well as their relationships to the cholinergic system. This may have important implications for normal function as well as dysfunction (such as presbycusis or tinnitus) associated with aging or injury and for the evolving design and use of cochlear implants and brainstem implants. From a training perspective, the proposed experiments cover a broad range of sensitive anatomical techniques, including a variety of multi-labeling methods and strategies for the effective combination of light and electron microscopy. This anatomical training is designed to expand and diversify the applicant's prior limited training in neuroanatomical techniques. The ability to combine and analyze multiple anatomical approaches will greatly expand the range of research questions that he will be able to address in the future.